It is well known to drive a staple through a pair of sheet materials in order to attach the sheet materials together. For example, staples are commonly used to attach pieces of paper together. In the manufacturing industries, it is known to attach together first and second sheet metal materials using a staple. A typical staple is made of a length of wire and has spaced apart legs that are driven through one of the sheets and then on through the other sheet. The legs may have a special configuration at the end thereof to facilitate the piercing and attaching operation. However, it would be desirable to provide further improvements in the design of such staples, particularly for the attachment together of steel and aluminum sheet materials.